Ex ovo omnia. Every multicellular body begins with an egg, which goes on over time to build the body in a process known as development. In this room, we have collected a variety of animal eggs, from the enormous ostrich egg to the invisibly small eggs of the mouse. Each of these eggs contains within it the power to give rise to a body capable of bursting out into the world as a new baby.

It all begins with an egg

Eggs can be round or elongated, large or small, hard-shelled or soft, black or transparent. Some are found in water, others on land, or inside the mother’s body. But despite all these seeming differences, all eggs are the same in one way: each is just a single cell.

Taking shape

All the cells in the body of the tiny roundworm can be seen as it develops into an adult. The fruit fly develops the segments of its body within the long capsule of the egg. The development of spiders and fish may look similar at first, but have important differences as well. Compare how embryos develop in these four organisms to learn about how bodies are built.

In the roundworm, cells quickly take on their final identities. The long egg of the fruit fly gradually transforms directly into sections that will become the head and the tail. In both zebrafish and spiders, development begins as groups of cells in a round egg divide and form migrating populations that give rise to head and tail. The central axis of the body forms on the back (dorsal) side in zebrafish, but on the belly (ventral) side in spiders. BRH highlights how things that seem the same may actually be different, and those that seem different, the same.

Born in the seas

The first animals emerged in the sea, and their babies needed to be born able to swim and fend for themselves right away. We explore evolution by looking at how such babies (called larvae) were able to move.
The larvae of many marine invertebrates are plankton, able to swim thanks to beating, hair-like cilia on their surfaces. The larva of the sea squirt is born with a muscular tail that enables it to swim, just as in the larvae of fish and amphibians.
From cilia to tails – some researchers believe that the changes in movement depending in these different structures may hold the key to understanding the transition from invertebrate to vertebrate life.

Eggs are busy places

An egg is just a single cell. It soon begins to divide and give rise to the cells of the body, which divide, adhere, and migrate during development. Soon structures like the belly and the head begin to emerge, as these “cells” become a “body.” Watch as the whole process plays out over time.
An egg appears much larger than other cells. But once fertilized, the egg begins to divide, and without changing its original size for some time, splits into many cells of ordinary size. This great power to give rise to all the cells of the body is truly why eggs are known as totipotent.

Spines and gills: The hallmarks of a vertebrate

A study spine is needed whether for animals to swim with their fins, walk on limbs or fly on their wings. As the spine develops, a small structure develops alongside in the neck region that is also important for animals moving on land or in the sea.
All vertebrate embryos, whether sea-dwelling or walking on all fours (tetrapods) develop a tissue called the pharyngeal pouch. In fish, this becomes the gills, while in tetrapods it gives rise to the middle ear that senses vibrations in the air, and the thyroid gland, which monitors and maintains calcium levels in the body. The same embryonic structure gives rise to specialized parts of the body that play roles appropriate to the animal’s environment.

Body segments keep it light: The arthropods

A light, strong exoskeleton, pairs of limbs on the left and right sides of the body that enable movement through water, on land or in the air. The repeated segments that make up the small bodies of arthropods, have given rise to the largest group of species on earth, and a remarkable diversity in form and function.
Around 80% of extant animals, or around 1 million species, are arthropods, a group whose great species diversity highlights its evolutionary success.
The arthropod body is made up of repeating segments. Limbs (or appendages) sprout from these segments in a variety of forms, depending on the species, enabling diverse approaches to life on earth.

Taking shape and facing the world

Every animal on earth undergoes embryonic development before leaving the egg or the mother’s womb to enter the world and grow. On reaching adulthood, it is now the animal’s turn to give rise to the next generation, thus enabling the continuity of life.Click here to see the enlarged image.